Ligand‐Field Splitting Parameter Optimization Achieves Synergistic Enhancement of Transition Metal Redox Activity and Structural Stability in High‐Voltage Sodium Layered Oxide Cathodes

Abstract Triggering oxygen anionic redox to achieve high‐capacity Na _x TMO ₂ faces a critical challenge because of the irreversible chemo‐mechanical distortion and uncontrollable oxygen release at high voltage. To circumvent this issue, a strategy of stimulating transition metal (TM) redox activity based on the ligand‐field splitting parameter ( Δ ) is proposed. Specifically, strongly polarized Mg−O−Fe configurations in the O3‐NaNi _0.1 Fe _0.2 Mn _0.5 Mg _0.2 O ₂ (O3‐NaNFMMO) is constructed to effectively optimize the electron occupancy state of Fe 3 d orbital by reducing its Δ , thereby stimulating the Fe redox activity while alleviating excessive oxygen redox. Additionally, the Mg pillar in Na sites ensures more extractable Na ⁺ and suppresses the Na‐free layers formation at high voltage, which can simultaneously improve the specific capacity and cycling stability. As a result, the designed cost‐effective O3‐NaNFMMO cathode delivers an outstanding specific capacity of 198 mAh g ⁻¹ at 0.1 C and high‐voltage cycling stability with 78% capacity retention after 1500 cycles at 5 C. Notably, the thermal degradation and air sensitivity, as the critical barriers to commercialization, are significantly suppressed in O3‐NaNFMMO cathode. This work establishes a universal design principle for high‐performance Na _x TMO ₂ cathodes and offers a scalable pathway toward practical, cost‐effective SIBs.